The present invention is directed to a reader for wirelessly receiving a signal from a tag and to a tag for wirelessly sending a signal to a reader. The present invention further relates to an efficient collision Recovery based on a modified tag and a modified reader.
In the recent years, the number of applications that use RADIO FREQUENCY IDENTIFICATION SYSTEMS (RFID) has increased, and the reading speed became one of the most critical issues in these applications. Such RFID networks comprise readers (interrogators), which are responsible of scanning the interrogation area and for identifying the tags. The RFID networks further comprise tags (transponders), which store the data to be read by the readers. In RFID systems, the tags are typically share a common communications signal. Thus, there is a certain probability of tag-collisions, i.e., multiple tags answer simultaneously. This collision probability naturally increases in dense networks with many passive tags. Due to the simple design of these passive tags, the reader is responsible for coordinating the network and has to avoid tags collisions using specific anti-collision algorithms. Amongst the RFID networks, attention shall be turned to Ultra High Frequency (UHF) networks which follow EPCGlobal Class 1 Gen 2 standards [1].
According to EPCGlobal Class 1 Gen 2 standards, the conventional anti-collision algorithm is Framed Slotted Aloha (FSA) algorithm which is only a Medium Access Control (MAC) layer protocol. In this algorithm, only the single tag replies (successful slot) are able to be decoded and then identified. Therefore, the conventional definition of the expected reading efficiency ηconv=P (1), where P (1)=n/L (1−1/L)n-1, wherein n presents the number of tags in the reading area and L is the frame length.
In the recent years, some research groups concentrated more to increase the reading efficiency through resolving the collided slots and convert them into successful slots. Shen et al. [2] proposed a collision recovery algorithm for the collided tags based on the signal constellations. However, he focused only on Low Frequency (LF) tags. Christoph Angerer [3], Kaitovic [4] and D. De Donno [5] have focused on the collision recovery of UHF tags. They have used the characteristics of the RFID signals to separate the signals from collisions at the physical layer. However, according to EPCGlobal Class 1 Gen 2 standards, the conventional reader is able to identify a single tag per slot at maximum. Moreover, the biggest problem in their work is the channel estimation mythology for more than two collided tags. Kaitovic [6] and [7] proposed an advanced channel estimation technique for the collided tags using an orthogonal post-preamble.
However, this technique is not compatible with the EPCGlobal Class 1 Gen 2 standards, which results to change the old system completely and build up a new standard.
According to EPCGlobal Class 1 Gen 2 standards, first of all, the reader broadcasts the frame size and notifies the beginning of a frame to all tags within a QUERY COMMAND. The frame size may include a number of slots to be used within the frame. After the frame is started, each tag generates a 16-bit random number (RN16) as a temporary ID and selects a slot in the frame. The tags count down the slot counters and back scatter their RN16 as soon as their slot counter is equal to zero. If there is a slot with no tag reply, the reader considers this slot as an empty slot and transmits a QueryRep command to decrement the tags slot counter. If only one tag responds to the slot, the reader transmits an acknowledge (ACK) command with the received RN16. Then, the tag replies with its electronic product code (EPC). If a collision occurs, and as illustrated in FIG. 14, there are two possibilities. First, in systems that have no capability of collision recovery, the reader queries the next slot by sending another QueryRep command.
FIG. 14 schematically illustrates different scenarios according to the known technology. According to scenario a, when the reader is not able to resolve a collision, both colliding signals from the tag comprising a respective RN16 are discarded. According to a scenario b, the reader may be configured for resolving a collision such that the signal 68a may be acknowledged by a respective signal 72 while the signal 68b is discarded. The tag may transmit a signal 74 comprising the EPC. Afterwards, the reader may transmit a query message 76 indicating that the next slot will start afterwards.
The maximum throughput of systems without collision recovery is 36% if the working frame length is equal to the tag population size. In systems that have a collision recovery capability, the reader transmits an ACK command to the tag with the strongest tag reply, i.e., the tag with the highest signal power. In this case, the tag which has the valid RN16 replies its EPC. The remaining tags forget their RN16 and wait for the next frame.